In optical communications systems, the use of wavelength selective switching for applications of optical cross-connects has attracted much interest because of the goal of fully flexible, networks where the paths of each wavelength can be reconfigured to allow arbitrary connection between nodes with the capacity appropriate for that link at a particular point in time. Although this goal is still valid, it is clear that optical networks will evolve to this level of sophistication in a number of stages—and the first stage of the evolution is likely to be that of a reconfigurable add/drop node where a number of channels can be dropped and added from the main path, whose number and wavelength can be varied over time—either as the network evolves or dynamically as the traffic demands vary.
This present invention is directed to applications such as reconfigurable optical add/drop multiplexer (ROADM) networks and is scalable to the application of wavelength reconfigurable cross-connects referred to generically as Wavelength Selective Switchs (WSS).
The characteristics of a wavelength selective element which is ideal for the applications of Optical Add/drop and Wavelength selective switching can be summarized follows:                i) scalable to multiple fibre ports        ii) one channel per port or multiple channels per port operation        iii) reconfiguration of wavelength selectivity to different grids eg/50 GHz or 100 GHz or a combination of both        iv) low optical impairment of the express path        v) low losses on the drop and express paths        vi) ability to add and drop wavelengths simultaneously        vii) ability to reconfigure between any ports or between any wavelengths without causing transient impairments to the other ports        viii) equalisation of optical power levels on express path (OADM) or all paths (WSS)        ix) provision of shared optical power between ports for a given wavelength (broadcast mode)        x) flat optical passband to prevent spectral narrowing        xi) power off configurations that leave the express path of an OADM undisturbed        xii) small power and voltage and size requirements.        
In reviewing the many technologies that have been applied it is necessary to generalize somewhat, but the following observations can be made.
Two basic approaches have been made for the OADM and WSS applications.                i) The first has been based on wavelength blocking elements combined with a broadcast and select architecture. This is an optical power intensive architecture, which can provide for channel equalization and reconfiguration of wavelength selectivity, but is not scalable to multiple ports, has very high loss and because of the many auxiliary components such as wavelength tuneable filters has a large power and footprint requirement.        ii) Wavelength switches have been proposed for OADMs, but do not naturally provide for channel equalization, the channel by channel switching in general leads to dispersion and loss narrowing of optical channels, and in the case of multiple port switches it is generally not possible to switch between ports without causing impairment (a hit) on intermediate ports. In addition the channel spacing cannot be dynamically reconfigured. Tuneable 3-port filters have also been proposed having a lack of impairment to the express paths but do not scale easily to multiple ports and may suffer from transient wavelength hits during tuning. Tuneable components are usually locked to a particular bandwidth which cannot be varied. In addition poor isolation of tuneable 3 ports means they are less applicable to many add/drop applications which demand high through path isolation.        
One technology that has been applied to optical cross connects has become known as 3-D MEMs utilises small mirror structures which act on a beam of light to direct it from one port to another. Examples of this art are provided in U.S. Pat. Nos. 5,960,133 and 6,501,877. The ports are usually arranged in a 2 dimensional matrix and a corresponding element of the 2 dimensional array of mirrors can tilt in two axis to couple between any one of the ports. Usually two arrays of these mirrors are required to couple the light efficiently and because of the high degree of analogue control required structures based on this technology have proved to be extremely difficult to realize in practice and there are few examples of commercially successful offerings. In this type of structure, a separate component is required to separate each wavelength division multiplexed (WDM) input fibre to corresponding single channel/single fibre inputs.
One of the most promising platforms for wavelength routing application relies on the principle of dispersing the channels spatially and operating on the different wavelengths, either with a switching element or attenuation element. These technologies are advantageous in that the switching element is integrated with the wavelength dispersive element—greatly simplifying the implementation. The trade-off is that in general the switching is more limited, with most implementation demonstrated to date being limited to small port counts—and the routing between ports is not arbitrary. In general a diffraction grating is used for micro-optic implementations or an Array waveguide grating for waveguide applications. Most of the switching applications have been based on MEMS micro mirrors fabricated in silicon and based on a tilt actuation in one dimension. The difficulty with this approach has been that to achieve the wavelength resolution required when the angular dispersion is mapped to a displacement. In such cases, an image of the fibre (with or without magnification) is mapped onto the tilt mirror array. In order to couple the light into a second port, additional optical elements are required that convert the angle into a displacement. Different approaches to this have included retroreflection cubes wedges (U.S. Pat. No. 6,097,519) which provide discrete displacements or Angle to Displacement elements (U.S. Pat. No. 6,560,000) which can provide continuous mapping using optical power provisioned at the Rayleigh length of the image. In all of these cases, in order to switch between ports, the tilt mirror needs to pass through the angles corresponding to intermediate ports. In addition, the number of ports is limited in each of these cases by the numerical aperture of the fiber as each of the different switch positions are discriminated by angles. For a fibre with a numerical aperture of 0.1, a switch which can tilt by +−12 degrees could not distinguish 8 different switch positions. One approach that can be used is to decrease the numerical aperture through the use of thermally expanded cores or micro lenses—but this is done at the expense of wavelength resolution.
An alternative has been to use polarization to switch between ports. Obviously this is most appropriate to switching between 2 ports corresponding to the 2 polarisation states—so is not readily scalable, though more complicated schemes can be envisaged to allow for switching between multiple ports. With polarization switching, the dynamic equalization of channels can be done at the expense of the rejected light being channelled into the second fibre—so it is not applicable to equalization of the express path whilst dropping a number of wavelengths.